This release took a while to bake in the oven, and there's a solid chance upcoming versions will take longer. After losing several key contributors to OnePlus, Paranoid Android's core team lacks the manpower it once had. It needs new muscle to keep the changes coming and welcomes contributions. In the meantime, the project is switching from a weekly release schedule to a "release when appropriate" timeframe.
Paranoid Android 5.1 is available for newer Nexus devices, including the Nexus 4. Sadly, the Nexus 10 hasn't made the list because the team doesn't have a device for testing. Nexus 6 builds offer WiFi calling on T-Mobile and VoLTE on T-Mobile/Verizon.
- Paranoid Android blog